The present invention relates generally to handling of large substrates. More particularly, the present invention relates to a method and apparatus for manipulating glass panels in the Flat Panel Display industry. Merely by way of example, the invention has been applied to gripping, squaring, and indexing glass panels during manufacturing and testing. But it would be recognized that the invention has a much broader range of applicability.
Large thin sheets of glass substrates used in the Flat Panel Display industry are commonly transported by floating on a pressurized air table (with or without preloading the substrate with vacuum) equipped with a matrix of air nozzles. The air cushion between the substrate and air table enables ease of manipulation and protection from damage during manipulation, provided that the air cushion is not compromised during the manipulation process by such effects as vibrations induced into the machine or poor mechanical handling while moving the substrate. Typically, vibrations are minimized by setting the air table within a highly rigid machine structure that includes costly precision parts and devices to minimize vibrations. Automatic mechanical handling of the glass is often achieved using several gripping devices, each of which includes a rigid vacuum pad to hold the substrate while squaring, aligning, indexing and transporting from the glass from one site to another during test. During these manipulations, the glass must remain floating at the nominal very small gap (˜200 microns) above the air table. Therefore, the gripping devices, which grab the glass, must also maintain the same nominal gap above the air table surface throughout the entire travel of the manipulation process. Otherwise, the gripper devices could drag the glass down along the air table surface and cause damage to both the glass and machine.
One known conventional gripper pad is mounted on a leaf spring for the required vertical compliance to ensure that the glass substrate remains floating above the air table. However, squaring the substrate requires rotating the substrate slightly, and the conventional gripper pad with leaf spring does not permit a rotational degree of freedom. Thus, use of such a conventional gripper requires handoff of the glass to a rotational cup to perform the squaring/alignment, and then a second handoff back to the leaf spring gripper to perform glass transportation (indexing). One shortcoming of this conventional gripper is that the leaf spring may be too stiff causing the gripper to easily lose grip, and potentially drag the glass along the air table.
Thus, such conventional, non-rotational gripping devices have the drawback of requiring a handoff process during which the glass must be held above the air table while being rotated (squared), and then returned to the gripper for X-Y translations. These several hand-offs can impact the machine cycle time and positional accuracy.